In some hydraulic networks wherein a liquid is supplied under the action of a pump, such as, for example, house water installations including a pressure water tank and water pump generally powered by an electric motor, the pump must be stopped down and started up according to the needs. In short, the matter is with the fact that the pump is to be started up when a valve or cock is opened, in order to hold a pressure in the tank and associated network, and stopped down when the cock is closed, in order to prevent a pressure rise over a desired value.
There are nowadays several apparatus for controlling the stop down and start up of such a pump.
The most used kind of apparatus consist of a pressure controlled switch connecting and disconnecting the pump within maximum and minimum pressure limits. In order to damp the hydraulic surge and also to act as a liquid accumulator thus lengthening the periods between the pump start up and stop down operations, the apparatus is provided with a so called "hydrosphere", a sphere shaped receptacle in the inside of which a movable membrane receives on one side the network liquid pressure and encloses at the other side a given volume of pressure air, the volume of which is variable according to the network pressure.
A safety detector must be installed together with the above apparatus, acting on the basis of a minimum level in the tank, in order to prevent the pump from burning down when it is sucking with no liquid in the network.
Besides, this kind of apparatus are subject to several drawbacks.
In first place, the hydrosphere is usually made of steel, which rusts thus causing impairment of the membrane and frequent replacement of this latter. In second place, the hydrosphere is very bulky, thus making difficult the installation thereof.
Owing to the constructive features of the apparatus, there is always a pressure variation between a minimum value and a maximum value, which is governed by a pressure controlled switch. This pressure variation may be rather high, for example of 2 to 3 Kg/cm.sup.2, and has a negative influence on the use of some apparatus such as the gas operated water heaters in which the lighting is controlled by the water pressure.
There are known also other kinds of apparatus essentially comprising a flow detector providing, in the absence of flow, the stop down signal for the pump, and a pressure detector providing, for a predetermined value of the pressure, the start up signal for the pump, and impeller means for the pressure detector, consisting of the hidrosphere membrane upon which the inner air acts.
In this case, some of the problems are solved because of the pump being started up by the flow detector, so that the pressure surge effect is considerably reduced. However, a hydrosphere is still present, though it may be smaller owing to the surge pressure effect being reduced. Some of the drawbacks which are inherent with the use of the hydrosphere are thus remaining: the air pressure within the hydrosphere is not easily regulable, and there is always some pressure loss with time, which claims for a periodic air replenishment.
On the other hand, the flow detector is associated with a valve which is operated by the liquid flow and provided with a return spring, and this latter performs closure somewhat abruptly, thus enhancing the pressure surge effect.